ABSTRACT The University of Chicago Medicine Comprehensive Cancer Center (UCCCC) Transgenic Mouse/Embryonic Stem Cell Facility (TMESCF) provides comprehensive genetic engineering services to alter the genome of the laboratory mouse. UCCCC investigators develop and exploit a variety of different mouse models of cancer to understand the etiology, progression and treatment of cancer as a disease. These approaches are particularly important to increase our understanding of the role of the immune system in tumorigenesis, and to develop increased knowledge of the factors affecting tumor metastasis, as well as other aspects of cancer that are best modeled in the whole animal. The Facility provides specialized technology to generate genetically engineered mice through the microinjection of mouse embryos and through the use of gene targeting/editing in embryonic stem (ES) cells. With the advent of new genome editing approaches, the demand for these services is greater than ever. In particular, demand from our faculty for CRISPR/Cas9 genome editing and TARGATT site- directed transgenics has grown exponentially over the past few years following our successful introduction of these technologies in 2013. The Transgenic Mouse/Embryonic Stem Cell Facility allows our investigators to develop mouse models of cancer at the cutting-edge of technology, with CRISPR/Cas9, for example, allowing us to introduce tags, specific deletions/mutations, and other genetic changes more rapidly and on a broader scale than in the past. Having such a dedicated Facility also allows our investigators to remain competitive in this rapidly evolving field, obtain much valued advice on the types of models that can be developed, and learn from the various new and innovative approaches being tested in the TMESCF.